1. Field of the Invention
This invention relates to a process for producing polyesters by ring-opening polymerization of lactones.
2. Description of the Prior Art
Roughly there are two types of polyesters obtained from lactones such as .epsilon.-caprolactone. One type includes polyesters ranging in form from wax-like solids to viscous liquids which are obtained by heating lactones together with organic compounds having a reactive hydrogen atom such as glycols or aminoalcohols (organic initiators). These polyesters are useful as raw materials for the synthesis of polyurethanes or as plasticizers for vinyl resins. Usually, polyesters of this type have a molecular weight of less than several thousand (e.g., about 300 to 7,000) (see Japanese Patent Publication No. 5293/59).
The other type includes solid polymers having a higher molecular weight. Polymers having a molecular weight of more than 12,000, unlike waxy lower-molecular-weight polymers, have high strength, and can be used as structural materials such as film coatings and adhesives including hot-melt adhesives. For example, poly-.epsilon.-caprolactone having a molecular weight of about 40,000, marketed under the trade name PCL-700 by Union Carbide Corporation, has a tensile strength of 3,000 to 4,000 psi (210 to 280 kg/cm.sup.2) and an elongation of 500 to 1,000% as described in a catalogue of that company.
The present invention relates to a process for producing these polyesters by the ring-opening polymerization of lactones.
It is well known that lower molecular weight polyesters useful as plasticizers or intermediates for production of polyurethane resins can be obtained by polymerizing lactones using compounds containing at least one active hydrogen atom such as a hydroxyl or amino group hydrogen atom, for example, glycols or amines, as initiators. In the production of such lactone polyesters, various organic acids, inorganic acids, metals, and metal compounds are used as catalysts.
Typical examples of metallic catalysts are organotin compounds or organic acid tin salts, such as dibutyltin oxide or tin octylate. Inorganic catalysts such as carbonates or oxides and metallic organic catalysts such as acetates and chelate compounds are also known which are derived from many other metals such as sodium, lithium, magnesium, aluminium, etc. For example, Japanese Patent Publication No. 5294/59 (corresponding to U.S. Pat. No. 2,878,236) discloses that chelate compounds of 18 metals which partly overlap the metals exemplified above can be used as catalysts in the production of lower molecular weight polyesters. As stated in the above-cited Japanese Patent Publication, to promote the reaction sufficiently, the polymerization catalyst is used in an amount of, for example, 0.05 to 0.1%, in the prior art. However, such a relatively large amount of the catalyst cannot be allowed to remain in lactone polyesters or it will affect the process for producing polyurethane resins in which the low molecular weight lactone polyesters find their main use. Specifically, such remnant catalyst tends to shorten the gelation time and thereby being harmful for handling and to impair the hydrolysis resistance, heat resistance, etc., of the resulting polyurethane resins. For this reason, the prior art has frequently required countermeasures such as the removal of the catalyst from the lactone polyesters or the addition of a masking agent.
Japanese Patent Publication No. 5293/59 discloses titanic acid esters (e.g., butyl titanate) as a catalyst suitable for obtaining polyesters of reduced coloration within short periods of time. However, this catalyst polymerizes under the influence of moisture and becomes inactive making it difficult to handle. Moreover, duplicated experiments have shown that this catalyst also has the disadvantage of tending to color polyesters.
Thus, known catalysts have not proven to be entirely satisfactory for the preparation of lactone polyesters which are colorless and have a low acid value.
Ring-opening polymerization of lactones in the presence of the above-exemplified catalysts is usually conducted to obtain low molecular weight polyesters in the presence of initiators as described above. If, in an attempt to obtain high molecular weight polyesters (having a molecular weight of more than 12,000), lactones are heated without using initiators in the presence of known catalysts other than the organometallic compounds described below, the polymerization conversion of the lactones is low, and the resulting polymers do not have a high molecular weight. For example, it is reported that a polycaprolactone obtained using potassium carbonate as a catalyst is a hard, brittle wax-like polymer having an average molecular weight of about 4,000 (Natta et al., J. Am. Chem. Soc., 56, 455, 1934). With many other catalysts, only low polymerization conversions can be obtained (see Comparative Examples hereinbelow).
Japanese Patent Publication Nos. 23917/65 (corresponding to U.S. Pat. No. 3,021,310), 26557/65 (corresponding to U.S. Pat. No. 3,021,309), 2473/68, and 14739/72 (corresponding to U.S. Pat. No. 3,632,669) disclose techniques for producing lactone polymers of high molecular weight. The average molecular weight of these polymers is from several hundred to several hundred thousand, for example, from 900 to 250,000 or more. One specific working example states that a highly crystalline fiber-forming solid polymer is obtained although its molecular weight is not specifically described. According to these references the catalysts used to obtain the lactone polymers are organometallic compounds such as phenyl magnesium bromide, butyl lithium, polyisobutyl aluminum oxide or dibutyl zinc. In these catalysts, the carbon atom of an organic radical is bonded directly to an aluminum atom or to a metal atom of Group I or Group II of the Periodic Table. These organometallic compounds have poor stability and lend themselves to difficult handling because upon contact with oxygen or moisture they immediately decompose or tend to burn. Furthermore, the amount of the catalyst used to obtain the lactone polymers is large (at least 0.3%), and the remainder of such a relatively large amount of the catalyst in the lactone polymers after the polymerization reaction tends to affect the heat resistance, etc., of the products. Thus, it has frequently been necessary to remove the remaining catalyst.